Introduction – In order to assure the performance of materials applied to wood, inspections of the cleaning and painting process should be undertaken. The general types of inspections are similar to those conducted when painting other substrates like concrete and steel.
Inspection of Cleaning and Painting – Wood is a dynamic substrate. In order for sealers or paints to provide the expected performance, the quality of cleaning, the conditions during application, and the application process need to be carefully monitored. Common inspection steps for assuring a quality installation are described below:
- Removal of degraded wood surface and deteriorated coatings if previously painted
Newly installed exterior wood surfaces will begin to degrade rather quickly. Ultraviolet rays can damage the wood surface within weeks. If the unpainted wood surface has been exposed to the elements for a long duration, the surface may be severely degraded or rotted. Degraded surfaces can cause coatings to fail prematurely if the surface is not adequately prepared prior to coating application. The USDA Forest Products Laboratory recommends that if the wood has been exposed for more than a few weeks, the surface should be lightly sanded or power washed to remove the thin layer of degraded wood. The degraded surface will have a grayish or dull appearance. When inspecting the surface after preparation, it is important to verify that wood rot is removed, the sanded surfaces exhibits an appearance like that of newer wood, and that the dust from sanding is removed by rinsing, vacuuming and/or wiping.
The condition of previously applied coatings must be examined before new coats are applied. Wood is a porous material that expands and contracts with moisture levels. The degree of expansion and contraction varies with environment (humidity), wood species and what part of the log the board was cut from. When existing coatings appear to be deteriorated, it is likely that they have lost elasticity and become brittle, losing the ability to expand and contract, leading to cracking, peeling, lifting, and loss of adhesion. When new coatings are applied to deteriorated coatings, the lack of adhesion and the brittleness of the existing coating will cause premature failure.
The specification should indicate whether some, or all, of the existing coating is to be removed. If only lose coatings are to be removed, a combination of scraping, sanding and pressure washing is typically involved. The inspection needs to verify that the poorly adherent coating has been removed. This is often done by probing the edges of the remaining coating with a dull putty knife to determine if it is intact. Depending on the circumstances, knife-cutting and probing can also be used.
If the specification requires that all coating be removed, thorough power sanding or a paint stripper might be involved. The inspection in this case is visual to confirm that the wood is bare and free of any coating, and in the case of stripper, that it has been completely removed. If the stripper was alkaline, pH paper can be used to verify that the surface is neutral (pH of 7) after rinsing. The inspection should also confirm that all surface dust has been removed prior to painting.
- Removal of surface contamination (grease, oil, dirt, chalking) and mold/mildew
A contaminated surface will greatly reduce the coating’s ability to adhere to the substrate or to the underlying coats. Warm water and scrubbing with a mild detergent will remove most grease, oil, dirt, and chalking. Heavy soiling may require several cleaning cycles. In all cases, the surface should be rinsed with clean water after scrubbing. The inspection of cleaning is typically visual and can be aided by wiping a cloth across the surface and inspecting it for contamination.
If biological growth such as mold and mildew are present, it can be removed by washing the surface with a solution of bleach and water. Ratios vary from 1 part liquid bleach to 3 parts water, to 1 cup bleach to 15 cups of water, which creates a gallon. The solution should be applied to the affected area, scrubbed with a stiff bristled brush, and allowed to remain on the surface for 10 minutes or until the growth is no longer visible. The surface should then be thoroughly rinsed with water and allowed to dry before painting. The inspection is visual to confirm that the biological growth has been removed. If there are questions whether an area contains growth or is just soiled, a few drops of bleach can be applied to the surface. If the darkness in the area in question does not disappear, addition cleaning should be undertaken. Adequate dryness for painting can be determined using instruments as described in section 3 below.
- Acceptable moisture content
Applying paint over wood surfaces with an excessively high moisture content can result in loss of adhesion, discoloration, peeling, blistering, and cracking. Acceptable moisture content for exterior wood surfaces prior to painting is no higher than 15%.
Pin and pinless meters are commonly used to determine the moisture content of wood. Pin-type meters use two contact pins that are pushed into the surface to measure the electrical resistance between the tips of the pins. Moisture content is determined at the depth to which the pins are pushed into the surface. The more water that is present in the wood, the less is the electrical resistance between the pins. The resistance is measured by the instrument and presented as percent moisture content.
For pinless meters, a scanning plate containing sensor pads is pressed against the surface. The instrument sends out electrical waves to create an electromagnetic field beneath the sensor pads. The amount of moisture in the wood effects the movement of the electrical impulses which is correlated to moisture content. The pinless meters generally measure moisture content from 0.25” to 0.75” from the surface.
Regardless the type of meter that is used, the moisture content should be checked in representative areas that will be painted. And note that because wood is hygroscopic, it gains or loses water moisture as the relative humidity (RH) of the surrounding air changes. Because of this, if the relative humidity levels are rising (when measured as described in #4), the moisture content in the wood is also rising and should be measured again.
Moisture content will obviously be high after a rain event and is also often high early in the morning. If the moisture content is above 15%, the wood should be allowed to dry further before applying paint.
- Proper ambient conditions for application and curing
Similar to substrates like steel or concrete, ambient condition requirements for the paint being applied must be met prior to applying paint or stain to wood surfaces. The specification or product data sheet will establish the acceptable air and surface temperatures, as well as dew point and humidity restrictions. Note that on hot sunny days, the surface temperature, especially when the color is dark, can exceed the maximum application temperature for the material and should be carefully monitored. The Forest Products Laboratory also recommends not painting a cool surface that will be heated by the sun within a few hours, or at the end of a cool day when heavy dew will form at night. The temperature must stay above 40°F (4°C) for 24 hours for oil paints and above 50°F (10°C) for latex. Some specifications may also require that the temperature of the paint be measured.
The ambient conditions should be taken prior to mixing and application, and at least every four hours during the work, or more frequently if the surrounding conditions change. Instruments such as a sling psychrometer are used to collect specific temperature values that are converted to relative humidity and dew point using charts or tables. A separate instrument is used to obtain the surface temperature. Alternatively, digital instruments are available that provide a direct read out of the ambient conditions and surface temperature.
Note that as the relative humidity increases, the moisture content of the wood is also increasing. If there is an appreciable increase in the relative humidity of the surrounding air, the moisture content of the wood as described in #3 should be measured again.
In addition to determining the ambient conditions, recoat times should be monitored when multiple coats are being applied. The recoat times are temperature dependent and can be found on the product data sheet.
- Mixing/thinning and storage of materials
Mixing and thinning of the materials should be monitored. The product data sheet will provide the mixing requirements. The inspection should verify that the shelf life has not expired, the correct mixing ratios and induction time of multiple component materials are observed, material temperatures are correct, only allowable thinning rates are used, and methods of mixing (hand mixing vs. mechanical mixing) are acceptable.
Many painting projects are performed in the summer when the ambient temperature is very hot. In addition to affecting the time of day that the paint can be applied, it can also affect where the paint can be stored. Most product data sheets provide storage temperature ranges that must be met. Storage of paint on the ground covered by a tarp may not comply. Air conditioned or even heated storage facilities may be needed. The paint storage conditions should be monitored.
- Proper application of materials
There are two primary types of coating material for wood surfaces: film forming finishes and penetrating finishes. The film forming finishes create a thin layer over the surface to which they are applied. The penetrating finishes absorb into the wood saturating the surface fibers and partially or completely filling the surface pores.
In general, either material can be applied by brush, roller or the use of spray equipment, but the product data sheet and specification should be consulted for any restrictions on methods of application.
Spray painting is the fastest way to apply the product but comes with a few disadvantages: preparation time to mask areas not to be painted, control of overspray, the potential for erratic thicknesses, and greater product loss. Even when spraying, follow up brushing or rolling may still be needed to work the product into the wood surfaces.
Brush and/or roller painting will be slower than spray application and may require more coats to achieve the specified thickness, but the painter will have greater control of the product being applied and the ability to work the paint into nooks and crannies.
One method for painting large expanses of wood is to have one applicator spray the product onto the surface with a second applicator following up with a brush and/or roller to work the paint into the surface.
- Proper coverage (spreading rate, WFT, DFT)
The specification or product data sheet provides the required dry film thickness (DFT) ranges for each coat as well as a spreading rate that correlates to the wet film thickness (WFT). The painters should be using wet film thickness gages to verify they are applying enough material to achieve the specified DFT range. Monitoring the wet film thickness and coverage rates are the common approaches for confirming that the correct amount of material has been applied.
Less common is the measurement of dry film thickness, but it can be done by using special ultrasonic gages, or by destructive testing. The use of the ultrasonic gages on wood is similar to using electronic gages for measuring coating thickness on steel substrates. The use of destructive tests is also similar to steel substrates with the Tooke Gage (a microscopic technique) being common.
- Film continuity and appearance (freedom from runs, sags, skips, misses, erratic gloss, etc.)
Like coatings applied to steel or concrete, coatings on wood surfaces should be uniform in appearance and free from runs, sags, skips, misses, erratic gloss, and other defects. The coating is a protective layer, so any skips or misses will allow moisture or other contaminants to enter the substrate potentially causing the wood to deteriorate and rot. Heavy runs and sags should be removed by sanding and coatings reapplied to the affected areas. During application, each coat should be closely inspected for imperfections so they can be repaired prior to application of the next coat.
Erratic gloss can be caused by over thinning, uneven application thickness, failure to prime a porous surface, and poor application techniques resulting in visible lapping by not keeping a “wet” edge.
Proper attention to application techniques and the skill of the painter can avoid these problems.
Conclusion – The performance of materials applied to wood substrates is enhanced as the quality of cleaning and painting improves. Inspection for compliance with specifications and good painting practice is a key component to improving performance.
The topics in wood series:
Topic 1 – Preparation of wood for painting
Topic 2 – Coatings and preservatives for wood
Topic 3 – Inspection of cleaning and painting